In future optical fiber communication systems it is likely that several optical carriers of different wavelength will be multiplexed onto a single fiber to increase its information carrying capacity. Recently wavelength multiplexers have been demonstrated which are compatible with multimode fibers. These multiplexers use graded index lenses and bulk frequency selective elements such as dielectric mirrors, prisms, and diffraction gratings. Several prior art integrated optical filters have been based on Bragg reflection in corrugated slab waveguides. These high reflectivity grating devices have generally been rather narrowband and not suitable for use with semiconductor lasers whose operating wavelength can drift over tens of angstroms.
One prior art filter which does have sufficient bandwidth is disclosed in U.S. Pat. No. 3,957,341, issued May 18, 1976 to H. F. Taylor, and entitled "Passive Frequency-Selective Optical Coupler." This filter by Taylor utilizes a directional coupler composed of two nonidentical strip waveguides which have intersecting dispersion diagrams. Wavelengths near the intersection of the dispersion diagrams, where the waveguides are phase matched, are able to transfer across to the other waveguide which provides the filtering function. The bandwidth of the filter is determined by the relative waveguide dispersion and the length of the coupler. Filters of this type can be contrasted to grating filters in that the filter structure inherently provides channel separation of the filtered light in a strip waveguide. The filter is also easily broadbanded. However, the filter as proposed by Taylor suffers from two drawbacks -- both the filter center wavelength and its crossover efficiency are subject to rather critical fabrication tolerances.